There have recently been proposals to allow access to the features and services provided by cellular networks, such as GSM and UMTS networks, other than by accessing those networks in the conventional manner. In this regard, the conventional manner is by signalling between a mobile terminal and a conventional base station (macro base station) that has a dedicated connection to a Mobile Switching Centre (MSC). It has been proposed to increase network capacity by providing additional special base stations, referred to as access points (APs), for example at a subscriber's home or office. Many different names have been given to APs, such as home access points (HAPs), HomeNodeB, micro base stations, pico-base stations, pico-cells and femto-cells, but all names refer to the same apparatus. APs provide short range, localized cellular telecommunications coverage, and are typically purchased by, or rented to, a subscriber to be installed in their house or business premises.
These types of base stations may be dedicated network access points, or may be enhanced wireless internet hubs (i.e. providing wireless internet access, as well as wireless telecommunications network access). The range of an AP is significantly smaller than macro base stations, typically only providing coverage of the order of 20 to 30 meters.
It has also been proposed to use APs in the Long Term Evolution (LTE) telecommunications network currently being developed, but not yet implemented. LTE is likely to be the next network implementation after 3G. Overall, the invention is applicable to any type of cellular telecommunications system, including GSM, 3G and LTE.
An advantage of introducing APs in existing telecommunications networks is that a higher quality of service can be offered to a customer, such that the AP can deliver higher throughput services at a lower cost. Further, where sufficient numbers of APs are implemented, the power level of the macro coverage could be reduced, due to a lower demand for the macro-base stations. Power reductions of course result in energy and financial savings, for instance due to less spectrum being required and also less hardware.
A further advantage of using an access point connected to the core network via an IP network is that existing broadband Digital Subscriber Line (DSL) connections can be used to link mobile terminals with the network core without using the capacity of the radio access network or transmission network of a mobile telecommunications network. In other words, the AP is integrated into a DSL modem/router and uses DSL to backhaul the traffic to the communication network.
A still further advantage is that APs are able to provide mobile network access to areas where there is no macro radio access network coverage. For example, an AP could provide 3G coverage in an area where there is no macro 3G coverage at all, perhaps only macro GSM coverage. The use of APs as an additional or alternative means for accessing the network therefore advantageously increases the network capacity.
However, additional challenges arise in implementing these access points in a well-integrated telecommunications network. For instance, in areas where one or more access points coexist with a macro base station (i.e. the access points and the macro base station both provide coverage to the given area), and where both an access point and the macro base station are operating on the same frequency channel, interference is an issue. In this regard, whilst access schemes like WCDMA use multiplexing to prevent users of the cell from interfering with each other, thereby providing intra-cell orthogonality, the same does not apply inter-cell. Therefore, users in an AP cell may interfere with neighbouring and/or overlapping macro-cells where the same carrier frequency is utilised, since users of each of these cells are not time and/or frequency multiplexed with respect to each others.
For instance, a mobile terminal (UE) using an AP may cause significant capacity reduction on the uplink on the surrounding macro cell when both the AP and the macro base station are using the same WCDMA carrier. Similarly a UE on the macro cell may degrade the performance of UEs using the AP. This problem is normally avoided by the operation of handover in which a UE will communicate with the base station which provides the best radio link. This will generally work in networks with only macro base stations. However in networks also including APs, an AP may not always permit a UE to handover to, or from, another base station and so performance degradation can result.
As an example situation of an AP not permitting handover, an AP may be a closed AP (i.e. only a dedicated list of subscribers are allowed on closed APs) and therefore not permit an unauthorised user to connect to it, even if it provides that user with the best available coverage. Therefore, in this situation the UE may not be connected on the best radio link, and as a result, the UE may increase its operating power in order to compensate, which will particularly affect an adjacent base station providing better coverage. This increase in operating power is likely to cause interference on the adjacent base station, which could be particularly acute if the power increase is large.
Further, the interference problem may occur when a UE is using an AP in a location where a macro base station provides good coverage, and the AP provides a lesser coverage. In this regard, it may be beneficial for the UE to stay connected to the AP, for instance to utilise a DSL connection to backhaul the traffic, or to make use of a preferential tariff offered when the AP is used. However, the lesser coverage of the AP, and the visibility of the macro base station offering good coverage is likely to result in the UE creating uplink interference to the macro base station.
Whilst one solution would be to operate the macro base stations on one carrier, and the micro base stations on another, spectrum is an expensive commodity, and it would not be financially viable to dedicate a whole frequency carrier for use by APs.
In this regard, it has been recognised that a major source of uplink interference is caused by UEs transmitting with excessive power in locations in which the UE is communicating with a base station, or AP, that provides a lesser degree of coverage than its neighbour.
There is therefore a need to provide an improved communication environment in order to allow base stations to better coexist on the same carrier. In particular, there is a need for an improved communication environment in order to allow macro base stations to better coexist with other smaller type of base station including APs on the same carrier.